The problem encountered with present star couplers is that they present relatively high insertion and division losses, which are functions of their order of division. These losses can be compensated by the presence of the above-mentioned amplifiers.
Conventionally, such amplifiers operate under saturation conditions in order to make high output powers available. This serves to correct emission power fluctuation at the other end of the link.
FIG. 1 shows the characteristic Ps=f(Pe) of an optical amplifier operating in saturation, and FIG. 2 shows a characteristic G=f(Pe) of such an optical amplifier where Pe is input power, Ps is output power, and G is gain expressed in dB. The characteristics are given for constant optical pumping power.
In FIG. 1, characteristic 10 shows that for low input power Pe, the output power Ps varies linearly. As the input power Pe increases, the output power Ps presents saturation. The gain G of the amplifier is given by Ps/Pe in the reference optical band, and thus by the slope of the characteristic 10. As mentioned above, the amplifier operates in saturation, and its operating point may be the point 11, for example.
FIG. 2 shows the gain G of an optical amplifier operating in saturation and as a function of input power Pe. The characteristic is referenced 20. As the power of the input signal increases, gain saturates and decreases. Gain compression is thus observed. The operating point is likewise referenced 11.
Optical amplifiers have a considerable response time: a change in input power over a time interval of less than about 10 ms will not be noticed by the amplifier, i.e. its output power will not vary. That is why the input power Pe and the output power Ps are taken into consideration as mean powers. Thus, it is considered that the input power Pe varies when the number of channels conveyed by the corresponding link changes. A change in the number of channels can be the result, in particular, of a drop or a rise in traffic, or of a change in allocation. It can thus be seen that a change in the number of channels gives rise to a change in the output power Ps of the amplifier.
Also, the entire link presents erratic optical noise (white noise). Thus, when only a few optical channels are to be amplified, the input power Pe is low but the gain G is at a maximum, so the noise reaching or created within the amplifier is amplified more than it would be with a larger number of channels to be amplified.
When amplifiers operating in saturation are used upstream from a star coupler, variations in the mean power load of each amplifier are observed as a function of variations in the number of channels conveyed and thus in the total mean power. These amplifiers therefore do not have the same gain and it follows that at the output from a star coupler combining various input ports over various output ports, the signal-to-noise (S/N) ratio of each channel and of each output port (and also its power level) is affected by variations of the load on one or more of the input ports. In other words, when a channel of a port is unused (no data to be transmitted or subscriber silence when transmitting a speech signal), the noise observed on each output port is greater than when that channel is in use. The problem is that a distributor element having a plurality of inputs such as a combiner or a star coupler, serves not only to split the working signal corresponding to a given wavelength and applied to a given input so as to distribute it over the outputs, but also to sum all of the noise levels present on all of the inputs, and the sum is likewise distributed over the various outputs.
One possible solution for remedying that problem (other than not amplifying the optical signals upstream from the distribution element, which would degrade S/N ratio) is to use amplifiers which are not saturated, i.e. in which the input power Pe does not give rise to gain compression.
By way of example, such amplifiers comprise respective short optical fibers subjected to high pumping flux. However, that solution is economically disadvantageous since the performance of such optical amplifiers is limited (gain of only about 10 dB) and they require high levels of pumping power.